1. Field of the Invention
The invention relates to a method of producing relief structures and somewhat more particularly to a method for producing relief structures comprised of double lacquer layers particularly on substrates already having relief structures for integrated semiconductor circuits wherein high-energy radiation, such as particulate or photon radiation, is utilized for structuring.
2. Prior Art
In order to be able to construct further structure levels on substrates already having relief structures, which are about 1 .mu.m high, it was heretofore necessary to coat the substrates with a negative resistant material at such a thickness that any steep relief structure edges were covered with the resist material at a sufficient thickness to adequately mask such edges, after irradiation and development, for subsequent etching. Typically, this resulted in lacquer or resist areas of different thicknesses on the substrate whereby the structure edges had the thinnest coating of resist. This caused a two-fold negative influence on the resist structure resolution during irradiation with high energy particles or photons, especially with electrons. Both the electron beam penetrating into such a resist layer and the electron beam significantly back-scattered at the substrate surface materially reduced the resist resolution power with increasing resist layer thicknesses. Accordingly, the limit of a resist layer applied to a planar substrate, which in this case resolved sub-micron structures, is, at least under certain conditions, displaced far into the micron range.
A double lacquer structure for integrated semiconductor circuits and method of producing such structures is suggested in U.S. Pat. No. 4,024,923. With this technique, at least two layers composed of polymers which chemically decompose upon irradiation and which are composed of materials that are so chemically different, that each layer can be developed without the other layer or without the other layer being significantly attacked by a developer for a given layer. The production of the lacquer or resist structures occurs in such a manner that the lacquer structure is irradiated through a mask which reproduces the desired pattern with radiation of a specific energy and intensity. In this manner, beginning with the uppermost polymer layer, all layers are developed in sequence and rinsing occurs after each development step. The uppermost layer is preferably composed of pure polymethyl methacrylate (PMMA), which is a positive lacquer or resist. This known double lacquer structure is utilized in the production of integrated semiconductor circuits for the so-called metal lift-off process and is intended to avoid both an insufficient structure edge coverage and a non-uniform structuring in the sub-micron range.
However, the foregoing known double lacquer structure and method of producing the same has a number of disadvantages. Among such disadvantages are (a) that the upper, positive lacquer layer required is relatively insensitive, (b) different developers or solvents must be utilized for the overall structuring process and this is uneconomical, and (c) the regulation of the development conditions are very critical.